Wet strength is a very important property for many grades of paper that are exposed to water during use. Grades of paper usually requiring wet strength include: bag, tag, toweling, tissue, map papers, paper pattern, napkins, ice cube bags, diaper liners, diaper wrap sheets, feminine napkin wrap sheets, disposable hospital bed pads, poster papers, filter papers, and many other grades of paper. Paper not treated for wet strength typically has 3-7% of its original dry strength available when tested while wet. Because of the need for paper products that retain some of their strength when soaked in water, chemical wet strength resins have been developed which produce paper products typically retaining 20-40% of their dry strength. In the paper industry, papers having wet tensile strengths of more than 15% of the original dry tensile strength are considered to be wet strength papers.
It is also possible to further subdivide wet strength papers based on the permanence of their strength when wet. Paper which has not been treated typically loses its strength within seconds of being soaked in water, while with some wet strength chemicals the rate of wet strength loss during soaking is slowed. Such papers are said to possess temporary wet strength. Other chemicals provide a longer lasting effect and are said to impart permanent wet strength, even though the wet strength is not fully permanent.
In order to achieve wet strength, wet strength resins have been developed which are, in general, chemically reactive, water-soluble polymers that are added at the wet end of the paper machine. They are typically quite expensive and are prone to a host of problems. The first resins to become popular in use for improving wet strength were the aminoplast resins, urea-formaldehyde and melanin-formaldehyde. These resins are thermosetting and require heat and low pH to properly cure. They had adverse effects on brightness and absorbency, and the low pH was corrosive to the equipment. They have fallen out of common use because of environmental problems associated with their formaldehyde content/release. More recently, epoxidized polyamide resins (PAE) and glyoxalated polyacrylamide resins have been developed and have found generally good acceptance in the paper industry. They can be used in neutral or alkaline conditions and, while the epoxidized polyamide resins produce a permanent wet strength, the glyoxalated polyacrylamide resins provide only temporary wet strength, although some increase in dry strength is also achieved. Despite the significant advances that these resins represent, there are still a great many problems associated with their usage including high cost, limited storage life, expensive addition systems, pH control on the paper machine, curing time, and sensitivity to other chemicals. In addition, the epoxidized polyamide resins have environmental concerns because of the absorbable organic halogen (AOX) emissions, while the glyoxalated polyacrylamide resins are not suitable for all uses because of the temporary nature of their wet strength development.
In this age of environmental awareness, the ideal wet strength agents are not currently available. Environmental concerns continue to influence research to develop new products that are more biodegradable and more compatible to a wide variety of ecological considerations while still accomplishing the task of providing suitable wet strength in the finished product.